In recent years, environmental issues have gained increasing significance, and CO2 emission controls have been tightened. Meanwhile, petroleum resources are limited, which may lead to difficulties in the supply of raw materials derived from petroleum resources in the future. In addition, petroleum prices are expected to soar due to a decrease in the supply every year. This creates a demand to replace raw materials derived from petroleum resources with raw materials derived from non-petroleum resources.
Conventionally, rubber compositions for tires generally contain aromatic oils as softeners. However, aromatic oils need to be replaced due to their carcinogenic properties and other problems. Today, tire companies in Japan are beginning to take measures to replace aromatic oils with various oils derived from petroleum resources (alternative aromatic oils) having a structure similar to that of aromatic oils.
However, alternative aromatic oils still depend on petroleum resources. Additionally, the addition of oil derived from petroleum resources in a rubber composition particularly containing natural rubber or butadiene rubber tends to increase rolling resistance of a tire (or deteriorate the performance in terms of rolling resistance) and increase fuel consumption (or deteriorate fuel economy). Moreover, whether aromatic oils or alternative aromatic oils are added, rubber compositions particularly containing natural rubber or butadiene rubber still leave room for improvement in terms of filler dispersibility as well as flex crack growth resistance and durability when used in sidewalls or base treads.
In this regard, for example, Patent Literature 1 discloses a rubber composition containing a vegetable oil (e.g. palm oil) which is used as a softener derived from non-petroleum resources to replace oil derived from petroleum resources. This rubber composition is excellent in terms of contribution to environmental friendliness but is very poor in filler dispersibility as well as in flex crack growth resistance and durability when used in sidewalls, compared with rubber compositions containing aromatic oils. As described above, a problem associated with the use of softeners derived from non-petroleum resources is that the flex crack growth resistance and durability provided by the addition of such softeners are merely equivalent to or less than those when conventional softeners derived from petroleum resources are added.